This invention relates to semiconductor lasers and more particularly to single and multiple quantum well lasers that may have their operating wavelength tuned to a shorter wavelength for particular applications requiring a principle operating frequency or emission wavelength within acceptable tolerances.
Semiconductor lasers are used in a wide variety of applications requiring desired and fairly precise intensity, optical output power and wavelength of operation. In fabricating a semiconductor laser by available epitaxial processes, it is not possible to know precisely what the predominant emission wavelength will be in the wavelength gain spectra of the laser. In the fabrication of quantum well lasers by metalorganic chemical vapor deposition (MO-CVD), it is possible to design the thickness of the active region comprising a single well or multiple wells and provide an operating wavelength within a particular range of predominant wavelengths, e.g., within 100 .ANG. to 300 .ANG..
Attempts have been made to design semiconductor lasers in such a manner that during their epitaxial growth various schemes can be introduced to hopefully obtain, within a reasonable tolerance, the desired emission wavelength. One such scheme, as indicated, is a predetermination of the thickness of the quantum well or wells and its composition e.g., GaAs or Ga.sub.1-x Al.sub.x As where x is fairly small. Another scheme is preselecting a dopant profile for growth of the multiple layers comprising the laser structure. A further scheme is the inclusion of a grating, such as in a distributed feedback (DFB) laser and a distributed Braff reflector (DBR) laser, wherein the grating period is chosen to obtain the desired emission wavelength from the wavelength gain spectra.
While these schemes provide an emission wavelength within a band of potential wavelengths, it is not possible, from a practical point of view, to provide a desired emission wavelength within, for example, .+-.10 .ANG.. This is particularly true because it is not known from the fabrication of one laser wafer run to the next run exactly what the resultant laser characteristics may actually be, principally due to the nonpredictability, in the fullest sense, of the interaction of the different process parameters and slight changes thereof occurring in the epitaxial processing.
What is needed is some process whereby the desired emission wavelength of the laser may be adjusted after laser fabrication.
One manner suggested for wavelength tuning of semiconductor lasers after fabrication is controllably altering the wavelength by applying a force to the laser structure, such as a mechanical stress in the form of hydrostatic pressure as taught in U.S. Pat. No. 3,482,189 or in the form of a sequentially applied current/voltage pulse scheme as taught in U.S. Pat. No. 3,312,910.
While these methods of wavelength alteration of lasers by subjecting the laser structure to an external force will vary the principal wavelength, they are not practical from the standpoint of permanent wavelength adjustment or change.